feat(dashboard): verification stats and auto-action summary

[jonesrussell]

Context

Depends on #327. With the AI worker running continuously, reviewers need visibility into what's happening automatically.

Requirements

• Stats banner at top of verification queue page:
  • Total pending (manual review needed)
  • Auto-verified count (last 24h / 7d)
  • Auto-rejected count (last 24h / 7d)
  • Average confidence score
• Requires a new API endpoint or extension of existing one

API (proposed)

GET /api/v1/verification/stats

Response:

{
  "pending_count": 12,
  "auto_verified_24h": 45,
  "auto_rejected_24h": 3,
  "avg_confidence": 0.87
}

Acceptance Criteria

• Stats endpoint added to source-manager
• Dashboard displays stats prominently on verification page
• Stats refresh on page load and after approve/reject actions